Future wireless systems require a more effective utilization of the radio frequency spectrum in order to increase the data rate achievable within a given transmission bandwidth. This can be accomplished by employing multiple transmit and receive antennas combined with signal processing, techniques commonly referred to as Multiple Input Multiple Output (MIMO) communication techniques. A number of recently developed techniques and emerging standards are based on employing multiple antennas at a base station and the mobile to improve the either the reliability of data communication over wireless media and/or the effective data rate delivered. These diversity/range (reliability) and multiplexing (throughput) benefits depend are dictated by the number of transmit and the number of receive antennas in the system and the type of transmission used. A system in which all transmit antennas and receive antennas are used to serve a single user are typically referred to as single-user MIMO systems.
Many existing and emerging wireless system standards supporting SU-MIMO. Here, for a given “resource block channel” in time and/or frequency, a scheduler chooses one among many users. For downlink communication, a point-to-point SU-MIMO transmission is used from the transmitter to the receiver of the scheduled user. Such a transmission is formed based on the encoding mode, selected often by the transmitter. This selection is based on feedback information provided by the scheduled user.
A number of systems around this general idea have been proposed for SU-MIMO based transmission. Some state-of-the-art schemes rely on providing high data rates via wideband transmission that relies on the use of OFDM. OFDM is attractive as it makes an equalizer unnecessary. With multilevel modems, coded modulation systems can be designed by means of an outer binary convolutional code and an interleaver in a so-called bit-interleaved coded modulation (BICM) system. Many state-of-the art systems employ coded OFDM/MIMO based transmission of this (or a similar) form, whereby the outer code and modulation scheme determine the rate of the transmission. Such a combination can be thought of as a transmission mode of the system.
The typical modes include an outer binary code with rate, indicated by a value Rc, followed by an interleaver and a mapper of groups of bits to complex valued symbols, adhering to a modulation scheme, e.g. a QAM constellation of size Q. In a BICM or D-BLAST type system, these symbols are passed in round robin fashion to the antennas for OFDM transmission over the appropriate resource block. Typically, the users are scheduled for transmission by looking at a channel quality level indicator (CQI), which often defines a simple measure such as the nominal received signal level. This helps determine the transmission rate to be used, as defined by the outer code's rate Rc and the modulation scheme.
In rate-adaptation schemes, it is important that the system rate is well matched to the channel's ability to support a certain rate. If not, an outage event will inevitably occur if a too high transmission rate is attempted. Although CQI values can prove some indicator of the achievable rates in single-input single-output (SISO) transmission, simple CQI feedback such as a nominal receive signal level are less meaningful for MIMO transmission.
Indeed, it is possible that two different MIMO channels with the same CQI level could support different rates over the channel. In other words, a CQI level indicator is in general insufficient in capturing the best rates that a specific modem can support over a MIMO channel. Indeed, even in the case that the CQI indicator is chosen as the capacity of the MIMO channel, which would seem to remove uncertainty in the supported rate, a given modem may only be able to support different rates on such distinct MIMO channels. That is, a given combination of Rc and Q, may behave differently on such distinct MIMO channels.
Finally, a receiver can always transmit full knowledge of the channel to the transmitter. This in general may require a very high feedback overhead. In addition, channel information itself may not sufficiently be able to determine actual receiver performance. Specifically, different receiver algorithms, even for the same channel and same modem, may behave very differently in terms of aspects such as bit-error performance and complexity. Thus, unless the transmitter has such knowledge of the receiver algorithm, it may not make the correct decision for a given mobile terminal having its own particular implementation of receivers. As a result, there is an inherent inefficiency in transmitter-driven rate adaptation schemes for SU-MIMO. It also does not support the ability of a single receiving terminal to have multiple potential receivers at its disposal, and to potentially choose in an adaptive way which receiver algorithm is used.